1. Field of the invention
The present invention relates to a fastening structure which fixes, with a holder, a plate-like member such as a doctor blade to maintain a certain thickness of a developer on the outer circumference of a developer roller of a develop unit such as a copier, a facsimile, or a printer. It also relates to a blade structure incorporating such a fastening structure, a develop unit incorporating such a blade structure, and an image formation apparatus incorporating such a develop unit.
2. Description of the Related Art
It is well known that an image formation apparatus as a printer comprises a develop unit (shown in FIG. 35, for example) which includes a blade structure composed of a developer roller to attract a two-component developer of a magnetic carrier and a toner or a one-component developer of a toner only onto its outer circumference to supply the developer to a latent image support body, as well as a doctor blade as a plate-like member to face the developer roller and maintain a certain thickness of the developer on the outer circumference of the developer roller.
A develop unit 100 shown in FIG. 35 includes a developer roller 101, a holder 102 facing the developer roller 101 and a doctor blade 103, and screw members 104 disposed with an interval in a longitudinal direction of the doctor blade 103. It is configured such that the doctor blade 103 is fastened on the holder 102 with the screw members 104 and brought in contact with the outer circumference of the developer roller 101 by a certain pressure F indicated by the arrow.
The doctor blade 103 is formed in a thin plate and functions to limit the amount of the developer on the outer circumference of the developer roller 101. The develop unit 100 in FIG. 35 is suitable for a one-component developer and limits the amount of the developer adsorbed on the outer circumference of the developer roller 101 by making the doctor blade 103 in contact therewith by pressure, for example.
In this type of develop unit 100, the doctor blade 103 is required to have uniform planarity or flatness because it has to evenly apply pressure to the developer roller 101 in an axial (longitudinal) direction thereof in order to control the amount of the developer on the developer roller 101 to be uniform in the axial direction.
However, the develop unit in FIG. 35 has a drawback that the doctor blade 103 exhibits great undulation and cannot attain designed straightness (linearity of a ridge line of the doctor blade in the longitudinal direction). This causes a non-uniform gap between the developer roller 101 and the doctor blade 103 in the longitudinal direction, resulting in insufficient friction to generate electric charges needed and formation of defective images.
Increasing the pressure F of the doctor blade 103 to the developer roller 101 may be a good way to solve this problem. However, with the increased pressure F, another problem may arise that the doctor blade 103 is likely to damage the developer roller 101 and shorten the longevity thereof.
In light of the above problems, Japanese Patent No. 3793370 (Document 1) discloses a blade structure to fasten a doctor blade onto a holder with a tightening torque gradually decreased from both ends to the center of the doctor blade.
Further, Japanese Laid-open Patent Application Publication No. 2003-280364 (Document 2) discloses a blade structure to fix a thin plate-like doctor blade to a holder by welding.
The inventors of the present application have proposed a fastening structure to fasten a thin plate-like doctor blade on a holder by plastic deformation of protrusions on the holder (disclosed in Japanese Laid-open Patent Application Publication No. 2007-245233 (Document 3), for example) and another fastening structure to fix a doctor blade to a holder by supporting it between the holder and a support plate (disclosed in Japanese Laid-open Patent Application Publication No. 2008-065011 (Document 4), for example).
Moreover, Japanese Laid-open Patent Application Publication Nos. 2001-129623 (Document 5) and No. 2006-126764 (Document 6), and Japanese Patent No. 3187103 (Document 7) disclose a blade structure to fix a doctor blade to a holder by plastic deformation of protrusions on the holder.
However, such blade structure and fastening structure still have their own problems. With the blade structure disclosed in Document 1, it is difficult to improve work efficiency since the doctor blade is fixed to the holder with plural screw members in a predetermined process while torques of the screw members are adjusted to gradually decrease from both ends of the holder to the center thereof.
As for the blade structure disclosed in Document 2, in order to prevent thermal distortion of the doctor blade by welding and maintain the straightness and planarity of the doctor blade, outputs of a welding machine are finely adjusted. However, there are some disadvantages that the output control of the welding machine is complex and difficult, requiring a great deal of time for manufacture preparation, and weld defects will occur due to variance in the outputs of the welding machine in mass production process.
Also, welding greatly increases manufacture cost and unit price of the blade structure. When the holder and the doctor blade are made of different materials, their linear expansion coefficients differ from each other, causing distortion and undulation in the doctor blade at welded portions and deteriorating the planarity thereof. Fixing the doctor blade to the holder by use of an adhesive faces a similar problem.
Further, the blade structure including the fastening structure disclosed in Document 4 uses the support plate for the purpose of preventing undulation in the doctor blade. The provision of the support plate disadvantageously increases the number of components and the number of assembly processes, leading to a complex components inventory control and a rise in manufacture cost for the blade structure.
For manufacturing a blade structure with use of the fastening structure in Document 3, it is necessary to make a cut in a convex portion of the holder with a punch (first pressing tool) when fixing a doctor blade to a holder. Force from the punch may adversely affect a flatness of the surface of the holder with which the doctor blade is overlapped, and deteriorates the planarity thereof, which leads to locally decreasing the planarity of the doctor blade pressed onto the holder. For thin doctor blades with low rigidness, this local low planarity may cause undulation in the edge of the doctor blade and disable the function of the blade structure.
Furthermore, the force from the punch making a cut in the convex portion may expand the bottom end of the convex portion. When the expanded convex portion comes in contact with the inner circumference of a through hole of the doctor blade, a great undulation will occur in the doctor blade, also disabling the function of the blade structure. A larger through hole can be a solution to this problem but at the same time it may cause insufficient strength for fastening the blade on the holder since an area of the cut portion to press the holder is narrowed.
Also, it is known that for fastening a thin plate in thickness of 0.5 mm or less on the holder without undulation in the plate, a force to sandwich the plate is necessary as when a safety razor is set to a holder.
For example, as shown in FIGS. 36, 37, a thin plate 105 is provided with through holes 106. When a force to expand the through holes 106 or the thin plate is applied to fasten them, undulation occurs in the thin plate by a little amount of force. It is known that in terms of undulation, a thin plate in thickness of 0.5 mm or less is unsusceptible to a force from a direction indicated by the arrows in FIG. 36 but susceptible to even a weak force from a direction indicated by the arrows in FIG. 37.
Given this fact, in manufacturing a blade structure by use of a metal plate with protrusions disclosed in Document 5, undulation may occur in a doctor blade due to a force arising from molding an expanded cylindrical portion. This may impair the function of the blade structure. Further, the undulation is conspicuous when the doctor blade as a plate-like member is in thickness of 0.5 mm or less.
There is another known method to fasten a plate-like member by making a cylindrical portion by extrusion, inserting a through hole of the plate-like member into the cylindrical portion, and then pressing down the cylindrical portion. However, such a method cannot also realize fastening the plate-like member without occurrence of undulation since expanding the cylindrical portion when fastening applies a force to expand the inner circumference of the through hole of the plate-like member.
The blade structures disclosed in Documents 6 and 7 cannot achieve fastening the thin plate without undulation either for the same reason as above.
Further, in case of molding a metal plate integrated with the cylindrical portion in Document 5, a very great force to place the metal plate between dies is needed. Otherwise, base materials of the cylindrical portion may not be protruded to form the cylindrical portion as it is supposed to be, and may flow to an outer circumferential direction, resulting in molding failures. In this case, the molding failures cause undulation in the doctor blade when attaching the blade to the holder.